The present invention relates to a backplane device for a light source array or for a light source matrix as well as a method of driving a backplane for a light source array or for a light source matrix. In particular, the present invention relates to a backplane device for an array or matrix of light sources being LEDs (Light Emitting Diodes) or OLEDs (Organic Light Emitting Diodes), which can be used for a display being suitable to display two-dimensional or three-dimensional information, images and scenes and video sequences. Backplane devices with an array or matrix of light sources being LEDs or OLEDs might be applied in particular for holographic display applications, e.g. like they are disclosed in WO 2006/066919 A1, which is incorporated by reference herewith. For such applications, a high frame rate might be needed, especially because virtual observer windows (VOW) might have to be generated in a time sequential manner for one or more observers. The backplane device according to the present invention can also be applied for a display being suitable to display three-dimensional information, images and scenes and video sequences in a stereoscopic or an autostereoscopic display.
OLEDs are used in television screens, computer monitors, small portable system screens such as mobile phones and PDAs, watches, advertising, information and indication. Due to their comparatively early stage of development, they typically emit less light per unit area than inorganic solid-state based LEDs similarly designed for use as point-light sources.
An OLED display functions without a backlight and so can display deep black levels and can be thinner and lighter than established liquid crystal displays. Similarly, in conditions of low ambient light such as dark rooms, an OLED screen can achieve a higher contrast ratio than either a LCD screen using cold cathode fluorescent lamps or the more recently developed LED backlight.
OLED displays can use either passive-matrix or active-matrix addressing schemes. Active-matrix OLEDs (AMOLED) require a thin-film transistor backplane to switch each individual pixel on or off, and can make higher resolution and larger size displays possible. In particular, the present invention relates to an AMOLED backplane device, e.g. especially an electric circuitry for driving and/or controlling an array or a matrix of OLEDs.
During operation, a voltage is applied across the OLED such that the anode is positive with respect to the cathode. A current of electrons flows through the device from cathode to anode, as electrons are injected into the lowest unoccupied molecular orbitals (LUMO) of the organic layer at the cathode and withdrawn from the highest occupied molecular orbitals (HOMO) at the anode. This latter process may also be described as the injection of electron holes into the HOMO. Electrostatic forces bring the electrons and the holes towards each other and they recombine forming an exciton, a bound state of the electron and hole. This happens closer to the emissive layer, because in organic semiconductors holes are generally more mobile than electrons. The decay of this excited state results in a relaxation of the energy levels of the electron, accompanied by emission of radiation whose frequency is in the visible region. The frequency of this radiation depends on the band gap of the material, in this case the difference in energy between the HOMO and LUMO.
For a high resolution display like a TV, a TFT backplane is necessary to drive the pixels correctly. Currently, Low Temperature Polycrystalline silicon LTPS-TFT is used for commercial AMOLED displays. LTPS-TFT has variation of the performance in a display, so various compensation circuits have been developed. Due to the size limitation of the excimer laser used for LTPS, the AMOLED size was limited. To cope with the hurdle related to the panel size, amorphous-silicon/microcrystalline-silicon backplanes have been reported with large display prototype demonstrations.
TFT backplane technology is a crucial enabler for the fabrication of flexible AMOLED displays. Two primary TFT backplane technologies (poly-Silicon (poly-Si) and amorphous-Silicon (a-Si)) are used today in AMOLEDs. These technologies offer the potential for fabricating the required active matrix backplanes at low temperatures (<150° C.) directly on the flexible plastic substrate for producing flexible AMOLED displays.
Passive-matrix OLED displays are now being used e.g. in mobile telephones. While conventional passive-matrix addressing simplifies the display fabrication, the number of rows is limited to a few hundred. Since the OLED is on only when being addressed, high peak currents are required to obtain average brightness levels. Row line resistance, column line resistance, and various OLED electrical characteristics restrict display luminance, size, format, and efficiency. However, for very-high-information-content displays, the cost of these approaches is likely to be prohibitive.
Thin-film-transistor (TFT) active-matrix backplanes can virtually eliminate the limitations of display content, size, format, luminance, and efficiency. Large-area high-resolution AMOLED TFT displays are being demonstrated with active-matrix TFT backplanes. One of the largest AMOLED display demonstrated to date, for example, uses a-Si TFT backplanes to achieve 20-inch diagonal HDTV formats with peak brightness (>500 cd/m2), with an efficiency >20 cd/A NTSC white. TFT active-matrix backplanes were initially developed for making large-sized and high resolution liquid-crystal displays (LCDs). The pixel circuit simply consists of a TFT connected to a storage capacitor and the pixel LC electrode. The impedance of the liquid crystal materials used is that of a capacitor whose value varies as a function of applied voltage as the refractive index changes. TFT performance is sufficient to stabilize the storage-capacitor voltage and LC voltage within a row time. The percentage of time that the pixel TFT is on and conducts is very low (˜0.1-1%). Applied data and LC voltages alternate polarity from frame to frame to avoid image sticking due to ion plating in the LC. The alternating data voltages and low duty factor on times tend to stabilize transistor characteristics such as threshold voltage for long operating lifetimes in AMLCDs.
Driving OLEDs uniformly with TFTs is more challenging than driving liquid crystal. The main reasons are (1) OLED current-dependent luminance or brightness, (2) large TFT dimensions with high gate-to-drain capacitance (Cgd) and gate-to-source capacitance (Cgs), and (3) threshold voltage and mobility variations. The drive TFT should provide a continuous current over a large portion of the frame time to efficiently drive the OLED to desired luminance levels. The pixel area limits the number of TFTs and their widths, which is directly proportional to TFT transconductance. As a result, the OLED driving TFT transconductance can be limited. The electron mobility (μ) of low-temperature polysilicon (LTPS) can be one to two orders of magnitude higher than that of amorphous-silicon (a-Si). As a consequence, LTPS TFT widths can be smaller, with possibilities of allowing for more TFTs in the pixel area for additional error correction. In addition, the LTPS TFT on-resistance may be lower, yielding better power-efficient operation. As a result of high gate capacitances, TFT on/off switching can create large voltage offsets. Thus, offset correction is required. OLED characteristics change with temperature rise caused by drive currents. This can result in luminance that depends upon the previous state. Pixel-to-pixel variations in Vt (threshold-voltage) and electron mobility p also add to unwanted luminance variations. With LTPS, initial Vt and mobility variations exist due to grain size and boundary variations. In contrast, in most a-Si processes, the initial Vt and p are uniform within a backplane. While time-related electrical stress may produce large Vt variations, there is typically little deviation in mobility. Optimized AC terminal voltages help to minimize time-related electrical stress variations.
Various techniques have been employed to minimize the impact of TFT variations with the use of simple pixel circuits. For example, restricting the use of an AMOLED display to video can assure that all pixels experience the same electrical stress. In one method, to obtain gray-level images, the bits are sequentially written with binary-weighted timing to the array. This requires a custom-designed frame buffer. In another method, the binary data bits are decoded to drive separate subpixel OLEDs. A lower cost solution is to send analog data to the pixel circuit and to have the driving method compensate for the Vt and electron mobility μ variations in the OLED driving TFT. Due to its inherent lower manufacturing cost, a-Si backplanes for driving OLEDs is of interest. Emphasis on lower cost also creates a need for simpler voltage-data circuits along with simpler driving methods.